It has long been known for electric furnaces to be used for melting scrap iron or similar ferrous raw material and reducing the resulting molten-metal bath, if appropriate with the addition of alloying elements, until a metal of specific composition is obtained.
In general terms, an electric furnace comprises a vessel limited by a sidewall and a bottom, covered with a hearth made of refractory material, and closed by a removable cover in the form of a vault, through which passes at least one electrode which is usually consumable and which consists of a graphite bar mounted so as to be vertically slidable, in such a way that it can descend within the furnace batch, normally scrap iron, which is in contact with at least one fixed electrode located in the hearth.
In the case of a single-phase alternating-current furnace and a direct-current furnace, the consumable electrode and the hearth electrode are connected to the two poles of a current source.
In a two-phase or three-phase alternating-current furnace, the consumable electrodes are connected to the poles of the current source, and the batch in contact with the hearth electrode constitutes the neutral of the system.
One or more electric arcs thus form between the batch and each consumable electrode, and these cause the melting of the scrap and the formation of a metal bath in the bottom of the shaft.
Until now, furnaces supplied with alternating current have been used by preference, but it was found that supplying the electrodes with direct current afforded many advantages, such as a reduction in noise and an increase in energy efficiency, because it is possible to use voltages higher than those possible with alternating current.
However, the use of very high intensities of direct current has until now been avoided, because, the currents always circulating in the same direction in the conductors, the electrodes and the bath generate considerable magnetic fields which deflect the arcs. To overcome this disadvantage, it has already been proposed to make the hearth electrode in the form of a plurality of conductive elements connected to the current source by means of one and the same return conductor and incorporated in the bottom, covering a zone of the latter of relatively large area centered relative to the vertical axis of the consumable electrode, so that the electric arc forms in the axis of this conductive zone. Moreover, in order to reduce the effects exerted on the arcs by the magnetic fields generated as a result of the circulation of current in the conductors connecting electrodes to the current source outside the furnace, it has also been proposed to surround the latter with a cylindrical screen made of magnetic metal and covering that part of the sidewall of the furnace which faces the current source, so as to form a magnetic core, rather than a "Faraday cage", interposed between the electrodes and the external conductors connected to the current source.
Such arrangements are often inadequate when a single consumble electrode arranged in the axis of the furnace is used.
But to obtain high electrical powers, it is expedient to pass the current through several vault electrodes, usually three, located above one or more hearth electrodes, thus making it possible to generate several electric arcs in the batch, but increasing the number of feed and return conductors generating magnetic fields and, consequently multiplying the deflection effects exerted on the arcs.
As long as the batch is in the form of scrap, the electrodes penetrate into this, at the same time digging in it pits which to a certain extent isolate the arcs from one another and promote their stability. In contrast, when the batch is completely melted, the arcs subjected to the magnetic effects generated as a result of the passage of the current in the electrodes, the conductors and other parts of the installation can form in unforeseeable directions and are therefore highly unstable.
The zone in which the arcs form and which is at the highest temperature consequently cannot be kept in the center of the furnace, the result of this being that the walls and the bottom can be subjected to excessive temperatures and to considerable wear of the refractory.